Alkaline cells have added mercury to the zinc anode to enhance current collecting effect by increasing contacts between zinc particles per se, and between zinc particles and current collectors. However, when mercury was removed from the zinc due to environmental concerns, performance and quality decreased. Basically, removing mercury from alkaline batteries impaired its leakproof characteristics, shock resistance characteristics, and discharge characteristics.
Many different approaches have been undertaken to overcome these problems. To alleviate the leakage problems associated with hydrogen gas generation, zinc alloy powders were produced with the addition of new metallic elements such as indium, bismuth, lead, aluminum and others. The problems of shock and vibration sensitivity were improved by the addition of gelling agents to the anode (such as carboxymethylcellulose, sodium polyacrylate, polyacrylic acid, for example). The gelling agents act to improve and increase contact points and firmness of the zinc particles. By using these gelling agents, the viscosity of the gel negative electrode increases to suppress the movement of the zinc particles. For example, European Patent EP 678927 teaches the use of three crosslinked gelling agents in combination for inhibiting vibration and improving shock resistance. This can impair discharge characteristics due to a poor reaction efficiency of the zinc and due to a reduction in electrical capacity ratio. Japanese published application JP 7-254405 teaches using a gelled negative electrode comprising non-amalgamated zinc powder in the shape of balls and long slender elements to increase contact points for improved dischargeability and flowability of the anode gel.
Due to the limited size of a battery's internal volume, battery manufacturers have been limited to the amount of active materials that can be packed into the cell. In order to provide the maximum electrochemical activity and a minimum of limiting polarization, it is desirable to operate a battery at as low a current density on the active materials as possible while still producing the required amount of total current from the system. Accordingly, alkaline batteries conventionally employ electrodes made from powdered active materials so as to obtain the highest possible surface area per unit weight or volume and thus minimize current density. The minimum amount of zinc powder in the anode needed to efficiently match the cathode's electrochemical potential has been about no less than 28 volume percent. Using higher amounts of zinc would unnecessarily waste zinc and restrict space around each zinc particle which limits the cell's solid reaction product capacity. The higher zinc content can therefore result in a decreased high rate service performance. Using lower amounts of zinc powders results in decreased electrochemical output and decreased voltage stability due to insufficient particle-to-particle and particle-to-collector contact.
Referring now to FIG. 1, conventional zinc powders used for alkaline batteries are shown as irregularly shaped particles, ranging from lumpy or distorted spheroids, to elongated tuberous forms. These particles frequently possess craggy, or minor protrusions, and irregular surface characteristics. The average surface area of these particles can be about 37 cm.sup.2 per gram of zinc. A particle's dimensions can be described in a three-dimensional system by its length, width, depth, thickness, and span dimensions. The span is most necessarily the same magnitude or orientation as the length. The aspect ratio of any particle is the ratio of its span to its width. The depth determines the minimum size sieve opening through which the particle will pass. The depth is not necessarily perpendicular to either the span or the length. For a typical battery grade zinc powder, the median depth, is determined by sieving, is approximately 100 to 300 microns; however, the extremes range from 20 microns to 1000 microns. Typical powders, as analyzed by scanning electron microscopy, have a depth that is nearly the same as the width and have aspect ratios of approximately 2 (i.e., the particles arc near spherical to elongated shapes). In conventional zinc powders, the ratio of the largest to the smallest lateral dimension is typically between about 1:1 to 10:1.
Nonetheless, until now, use of conventional zinc powders has required no less than 28 volume percent of zinc in the anode to provide sufficient electronic conductivity. Accordingly, it is desirable to have a zinc anode that enables significantly lower amounts of zinc to be used in the anode, while still maintaining an adequate current carrying matrix while maintaining good conductivity, improved high current discharge efficiency, and solves the problem of shock and vibration sensitivity.